Vertical spacing in text style fractions with Stix 2 fonts

Question

I am using the Stix 2 fonts for some of my documents and am pretty happy with them except for one thing: I feel they place an excessive amount of vertical spacing around fraction lines in text style math. Here's a comparison to Latin Modern.

% !TeX program = lualatex
\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\newfontfamily\lmodern{LatinModernRoman}
\newfontfamily\stixii{StixTwoText}
\setmathfont{LatinModernMath}[version=lmodern]
\setmathfont{STIXTwoMath}[version=stix2]
\NewDocumentEnvironment{comparison}{ O{.48\textwidth} +b }{%
  \par
  \begin{minipage}{#1}%
    \lmodern
    \mathversion{lmodern}%
    \raggedleft
    #2%
  \end{minipage}\hfil
  \begin{minipage}{#1}%
    \stixii
    \mathversion{stix2}%
    \raggedright
    #2%
  \end{minipage}%
  \par\vskip 1ex\relax
}{}
\begin{document}\centering
\begin{comparison}
  Latin Modern vs Stix~2
\end{comparison}
\begin{comparison}
  (\frac HH)
\end{comparison}
\begin{comparison}
  (\displaystyle
    \frac{ \frac HH }{ H + H }
  )
\end{comparison}
\begin{comparison}
  (\displaystyle
    H^{\frac HH}
  )
\end{comparison}
\end{document}

This leads to fractions being pretty much unusable in in-line math, which one might argue is a good thing (though I do not agree). But even in display math I find them taking up an unreasonable amount of space when appearing in nested fractions of in exponents. Note how the "middle" H in the nested fraction is closer to the big fraction line than to the small one. This just looks weird.

So my question is twofold:

• Is this really intended? What might be the reasons for making it so?
• Do I have any recourse for changing this without changing fonts? That is, is there some (La)TeX dimension I can adjust to reduce this space?

Answer 1

As in classic tex you can fiddle with the parameters, here I reduce the spacing and increase the rule width (it's subtle, but you may notice the difference)

% !TeX program = lualatex
\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\newfontfamily\lmodern{LatinModernRoman}
\newfontfamily\stixii{StixTwoText}
\setmathfont{LatinModernMath}[version=lmodern]
\setmathfont{STIXTwoMath}[version=stix2]
\NewDocumentEnvironment{comparison}{ O{.48\textwidth} +b }{%
  \par
  \begin{minipage}{#1}%
    \lmodern
    \mathversion{lmodern}%
    \raggedleft
    #2%
  \end{minipage}\hfil
  \begin{minipage}{#1}%
    \stixii
    \mathversion{stix2}%
    \sbox0{$$}% wake up math settings
    \z\raggedright
    #2%
  \end{minipage}%
  \par\vskip 1ex\relax
}{}
\def\z{
\Umathfractionnumvgap\displaystyle=0pt
\Umathfractionnumvgap\textstyle=0pt
\Umathfractionnumvgap\scriptstyle=0pt
\Umathfractionnumup\displaystyle=0pt
\Umathfractionnumup\textstyle=0pt
\Umathfractionnumup\scriptstyle=0pt
\Umathfractionrule\displaystyle=25pt
\Umathfractionrule\textstyle=25pt
\Umathfractionrule\scriptstyle=25pt
}
\begin{document}\centering
\begin{comparison}
  Latin Modern vs Stix~2
\end{comparison}
\begin{comparison}
  (\frac HH)
\end{comparison}
\begin{comparison}
  (\displaystyle
    \frac{ \frac HH }{ H + H }
  )
\end{comparison}
\begin{comparison}
  (\displaystyle
    H^{\frac HH}
  )
\end{comparison}
\end{document}

Answer 2

For anyone looking to do the same: Here are the settings I settled on following David's answer.

% !TeX program = lualatex
\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\newfontfamily\lmodern{LatinModernRoman}
\newfontfamily\stixii{StixTwoText}
\setmathfont{LatinModernMath}[version=lmodern]
\setmathfont{STIXTwoMath}[version=stix2]
\NewDocumentEnvironment{comparison}{ O{.48\textwidth} +b }{%
  \par
  \begin{minipage}{#1}%
    \lmodern
    \mathversion{lmodern}%
    \raggedleft
    #2%
  \end{minipage}\hfil
  \begin{minipage}{#1}%
    \stixii
    \mathversion{stix2}%
    \raggedright
    \stixiifontpatch
    #2%
  \end{minipage}%
  \par\vskip 1ex\relax
}{}
\makeatletter
  \def\scaleluamathparam#1#2#3{%
    @tempdima=#1#2%
    #1#2=#3@tempdima
  }
\makeatother
\def\stixiifontpatch{%
  \sbox0{()}% wake up math settings
  \scaleluamathparam\Umathfractionnumup\textstyle{.7}%
  \scaleluamathparam\Umathfractionnumup\crampedtextstyle{.7}%
  \scaleluamathparam\Umathfractionnumup\scriptstyle{.7}%
  \scaleluamathparam\Umathfractionnumup\crampedscriptstyle{.7}%
  \scaleluamathparam\Umathfractionnumup\scriptscriptstyle{.7}%
  \scaleluamathparam\Umathfractionnumup\crampedscriptscriptstyle{.7}%
  \scaleluamathparam\Umathfractiondenomdown\textstyle{.7}%
  \scaleluamathparam\Umathfractiondenomdown\crampedtextstyle{.7}%
  \scaleluamathparam\Umathfractiondenomdown\scriptstyle{.7}%
  \scaleluamathparam\Umathfractiondenomdown\crampedscriptstyle{.7}%
  \scaleluamathparam\Umathfractiondenomdown\scriptscriptstyle{.7}%
  \scaleluamathparam\Umathfractiondenomdown\crampedscriptscriptstyle{.7}%
}
\usepackage{mathtools}
\usepackage{xfrac}
\begin{document}\centering
\begin{comparison}
  Latin Modern vs Stix~2
\end{comparison}
\begin{comparison}
  (\displaystyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\begin{comparison}
  (\crampeddisplaystyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\begin{comparison}
  (\textstyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\begin{comparison}
  (\crampedtextstyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\begin{comparison}
  (\scriptstyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\begin{comparison}
  (\crampedscriptstyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\begin{comparison}
  (\scriptscriptstyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\begin{comparison}
  (\crampedscriptscriptstyle\frac HH\frac{H_H^H}{H_H^H})
\end{comparison}
\end{document}